Technology

The 10B(n,alpha) reaction and detector design

The boron-coated straw (BCS) technology is built on a patented low-cost technology that places long copper tubes (straws) of variable diameter, coated on the inside with a thin layer of 10B-enriched boron carbide (10B4C). Thermal neutrons captured in 10B are converted into secondary particles, through the 10B(n,α) reaction. The secondary 7Li and α (or 4He nucleus) particles are emitted isotropically in opposite directions with kinetic energies of 1.47 MeV and 0.84 MeV, respectively (dictated by the conservation of energy and momentum). For a boron carbide layer that is only 1 μm thick, one of the two charged particles will escape the wall 78% of the time, and ionize the gas contained within the straw. Each BCS is operated as a proportional counter, with the tube wall acting as the cathode, and a thin wire tensioned through its center serving as the anode electrode, operated at a high positive potential. Primary electrons liberated in the gas drift to the anode, and in the high electric field close to the anode, avalanche multiplication occurs. This avalanche delivers an amplified charge on the anode wire, which is the detected signal. Standard charge sensitive preamplifier and shaping circuitry are used to produce a low-noise pulse for each neutron event. Gamma interactions in the wall produce near minimum ionizing electrons that deposit a small fraction of the energy of the heavily ionizing alpha and Li products. Gamma signals are effectively discriminated with a simple pulse height threshold.

The detector design, as just discussed, consists primarily of a BCS inside an aluminum tube with a tensioned wire down the shared axis of the tube and BCS. Endcaps are laser welded onto the tube to form a hermetic seal, and the working gas that supports the avalanche multiplication is flushed into the tube using the gas port that is then sealed. In comparison to 3He-based detectors, which require multiple atm of pressure to maintain high sensitivity, 1 atm or less is sufficient for any BCS sensitivity. For cases where greater sensitivity is required in a BCS-based detector, BCS with unique cross sections and/or multiple straws can be placed inside a single aluminum tube, effectively increasing the density of 10B inside the tube.

The impact of BCS detectors

BCS detectors are similar to 3He detectors in all the ways that matter: their shape facilitates straight forward 1:1 replacement of 3He detectors, the detected signal is proportional to the energy of the neutron, and both have high neutron capture cross sections providing for high sensitivity. Yet BCS detectors improve upon 3He in several ways, making them significantly more versatile in their applications.

Naturally, BCS detectors that rely on 10B will be significantly cheaper and more widely available than 3He because 10B is a highly abundant resource. Not only does BCS technology employ a more abundant resource than 3He, but it is also safer and more reliable in the long term compared to other 3He replacement technologies. BCS are safer to work with than manufacture than 10BF3 gaseous detectors while avoiding many of the issues that 10BF3 detectors have in common with 3 B4C enrichment with 10B is more environmentally friendly than 6Li enrichment for6Li scintillators and doesn’t have the same detector aging issues that 6Li scintillators do.

BCS detectors can reach significantly smaller sizes, because the secondary particles resulting from neutron capture in 3He travel significantly longer distances than for 10B. When those secondary particles collide with the detector wall, they cannot deposit their energy into the counting gas and therefore the signal is not counted for that event.

BCS detectors have an advantage in multiplicity or coincidence counting for identification of special nuclear materials. Only one of the secondary particles will escape the wall and be counted, as compared to 3He detectors which capture neutrons in the gas volume of the detector, ensuring by definition that both secondary particles will be counted. In this way, BCS detectors have significantly reduced error on the timestamp associated with each neutron event, making it easier to measure neutron multiplicity.

BCS do not require high-pressure working gas to operate efficiently, which makes them safer, easier to work with, and significantly lighter than their 3He counterparts. BCS detectors are straight forward to transport and, should catastrophic failure occur, they fail safely without explosion. BCS require no maintenance of any high pressure and high purity working gas as in a 3He detector. BCS do not need thick metal walls to maintain pressure and prevent leaks, which makes it possible to group the detectors closer together for improved spatial coverage and sensitivity.